Conventional autodyne motion sensors used to actuate automatic doors generate a phase coherent beam, typically of X or K-band microwaves, that is reflected from moving objects within the field of the sensor back into the sensor. The received microwave reflections are electronically compared within the sensor with the outgoing microwave signal (autodyne process) to create a difference Doppler signal proportional to the amplitude and relative phase of the reflected microwaves. The frequency of this Doppler signal (Doppler shift), which is typically in the low audio range, is proportional to the component of the motion of the reflecting object toward or away from the source of the microwaves within the sensor (longitudinal motion). The Doppler signal is processed by a variety of means into a binary motion detection signal that is used to actuate the automatic door.
Motion sensors of this design suffer from a number of significant shortcomings. First, they respond poorly, or not at all, to objects moving at right angles to the line of position between the beam source within the sensor and the reflecting object (transverse motion). This is because instantaneously the relative distance between the sensor and the object does not change for this type of motion. Under this circumstance the frequency of the Doppler signal drops to zero and, in conventional designs, is undetectable. Consequently, to be effective and reliable, motion sensors of this type must be positioned so that the direction of the anticipated motion to be detected is not purely transverse but contains a substantial longitudinal component.
Second, motion sensors of this design are not able to discriminate between the motion of the door they actuate and pedestrians and other objects passing through the door. Consequently, they must be deployed with their beams aimed generally away from the door so that they respond only to pedestrians and objects approaching the door but not to the door itself. Otherwise, the motion of the door will trigger the sensor, resulting in endless cycling of the door. This consideration limits possible sensor mounting positions to areas immediately adjacent to the door, such as on the door header or jambs. To minimize contact, access or interference with pedestrians or objects passing through the door, this type of motion sensor is invariably mounted on the door header facing generally toward the approaching traffic.
As a result of the foregoing, motion sensors of this design must be positioned more-or-less directly above a pedestrian or object about to enter the area swept by the door. Because the motion of pedestrians and objects passing through the door is substantially horizontal and, consequently, transverse to the motion sensor on the door header, the Doppler shift, upon which the sensor depends to generate a motion detection signal, is minimal or zero in the region just in front of the door opening. Furthermore, in order to prevent triggering of the sensor by the motion of the door itself, the motion sensor must be aimed so that the proximal operational edge of its detection beam or pattern lies just in front of the door opening, coincident with the area of reduced detection capability due to minimal or zero Doppler shift. The edge of the detection beam is defined as the locus of points outside of which the detection capability of the sensor drops to zero due to the fall off of the beam intensity, independent of the Doppler shift. Consequently, motion sensors of this design exhibit severely compromised detection capability immediately in front of the area swept by the door due to the unfortunate coincidence of the minimal Doppler shift and beam edge effects in that region.
In many door designs, other sensors that might prevent door motion when an object or person is present must be disabled for part of the door cycle, leaving the area swept by the door unprotected. Such designs depend upon the motion sensor, which is not disabled, to detect residual motion of pedestrians or objects into the unprotected door opening. Because of the compromised detection capability of motion sensors of the type considered immediately in front of the door opening as explained above, motion detection may fail, with the result that a person or object may enter the sweep of the door and be struck.
A further disadvantage of motion sensors of conventional design is that, because they must be aimed generally away from the door that they control, they provide incomplete coverage in the area immediately in front of the door opening. In particular, they provide poor coverage in the vicinity of the door jambs. This is because the area within which they detect is bounded by a roughly elliptical perimeter. Consequently, the detection area, which closely approaches the door opening on the centerline through the doorway, necessarily curves away from the plane of the door opening in either direction to the side away from the centerline. In contrast, the older control mat technology, which beam sensors have largely replaced, provides uniform coverage all the way across the door opening, right up to the door jambs.
A further shortcoming of current autodyne motion sensor designs is that they do not take full advantage of the phase information that can be extracted from the energy reflected back into the sensor. Many designs contain only one demodulator, so that, in any case, only half of the phase information is available. [The term "demodulator" is used throughout this document in place of the customary "detector" to differentiate between the electronic detector component, or components, within the sensor and the overall function of the sensor itself as a detector of motion.] Designs that do contain two demodulators use the available phase information only to determine whether the detected object is moving toward or away from the sensor. The individual signals from the demodulators, whether one or two, are oscillatory. To minimize false triggering, the motion detection signal in conventional designs is generated only when a certain number of cycles of this oscillatory behavior occur within a prescribed interval of time. As a result, there is an inherent time delay between the onset of oscillatory behavior and generation of the motion detection signal. On the other hand, the delay can be reduced considerably, without incurring a higher frequency of false detections or sacrificing the ability to discriminate motion toward or away from the sensor, by properly combining the signals from two demodulators.
It would be desirable if motion sensors of the autodyne type could be mounted in locations other than on the door header, thereby removing the region of minimum Doppler shift from the area immediately in front of the door opening. However, this would necessarily require that the motion sensor beam be directed generally toward, rather than away from, the door opening with the result that, with current designs, the door would be endlessly cyclically actuated. Thus, it would also be desirable if the motion sensor could be designed selectively to ignore the signature of the door itself while remaining fully capable of responding to the motion of objects other than the door. A sensor so designed could then be deployed with its beam facing generally toward the door, removing both the beam edge and the region of minimal Doppler shift from the area immediately in front of the door opening. Furthermore, as a consequence of being aimed toward the door, the detection area of such a motion sensor could extend all the way to the door jambs, unlike current designs. It would also be desirable for the motion sensor to be designed to combine the phase information available from two demodulators in quadrature to minimize the delay in generating the motion detection signal without increasing the frequency of false motion detection.